Method of coating a moving metal strip

ABSTRACT

A method of coating a moving metal surface comprising forming a dry layer of metallic powder on the surface, thereafter applying an aqueous liquid of a substance which gels when suitably hydrated to the said layer and allowing the water to penetrate the metallic powder layer, and then passing the metallic powder layer against a compression roller to compact the powder to the said surface, the said liquid acting to reduce the extent to which the roller dislodges the powder.

United States Patent Jackson et al. 45 M 9, 1972 [54] METHOD OF COATING A MOVING 1 References Cited METAL STRIP UNITED STATES PATENTS [72] Inventors; Albert Edward Jack on wendover G 2,178,529 10/1939 Calkins et a1. 1.01 17/22 ffield Mold; p m Haydn Hughes, 2,877,738 3/1959 Heck T. ..,.1 17/16 Wrexham, both of E l d 3,231,971 2/1966 McFarland et a1... v...l 17722 I 3,364,057 1/1968 Jackson 1117/22 1 1 Assigneer Johqsumflfers Sons LII/"fled, 5110mm 3,382,085 5/1968 Wren etal. ..117/e5.2 Deesldel Flmlshlre, England 3,428,472 2/1969 Shimose et a1... ....1 17/22 22 Filed: 9,1968 3,485,654 12/1969 McGraw Ct 211. 1 17/17 1,823,869 9/1931 Baur ..1 17/22 [21] App1.No.: 782,259

Primary Examiner-William D. Martin Assistant Examiner-Raymond M. Speer {30} Foreign Application Priority Data Anomey Mawhinney & Mawhinney Dec. 14, 1967 Great Britain ..56,871/67 [57] ABSTRACT [52] 1 17/26 A method of coating a moving metal surface comprising form l 11 l 1174131, ing a dry layer of metallic powder on the surface, thereafter Int Cl applying an aqueous liquid of a substance which gels when [58] Field of Search ..1 17/16, 17, 21, 22, 26, 29, sultably hydrated to the Said layer and allowmg the water to penetrate the metallic powder layer, and then passing the metallic powder layer against a compression roller to compact the powder to the said surface, the said liquid acting to reduce the extent to which the roller dislodges the powder 13 Claims, 1 Drawing Figure METHOD OF COATING A MOVING METAL STRIP This invention concerns a method of coating a moving metal surface, particularly a moving metal strip.

According to the present invention, there is provided a method of coating a moving metal surface comprising forming a dry layer of metallic powder on the surface, thereafter applying a liquid to the said layer, and then passing the layer against a compression roller to compact the powder to the said surface, the said liquid being such as to reduce the extent to which the roller dislodges the powder.

The liquid may glaze the surface of the layer to reduce the extent to which the roller dislodges the powder.

The liquid may contain a metal hydroxide.

The said surface may be the surface of a moving metal strip there being provided a pair of co-operating said compression rollers through the nip of which the strip is passed.

Preferably the layer is dried after the liquid is applied, before being passed against the said roller.

The said liquid may be an aqueous colloidal solution of a substance which gels when suitably hydrated.

The liquid may be an aqueous colloidal cellusic solution.

Thus the liquid may be an aqueous solution containing sodium carboxy methyl cellulose.

Preferably the concentration of the sodium carboxy methyl cellulose in the solution is not less than 0.05 percent nor more than 0.6 percent by weight.

In a preferred embodiment, the said concentration is not less than 0.2 percent nor more than 0.4 percent.

Alternatively the solution may contain starch, at a concentration between 0.2 and 1 percent by weight.

Alternatively the solution may contain Bentonite, at a concentration of not less than 0.5 percent nor more than 2 percent by weight.

Alternatively the solution may contain aluminum hydroxide or nickel hydroxide at a concentration of not less than 0.05 percent nor more than 1 percent by weight.

Preferably the liquid is applied to the surface at a rate between 2 and 4 cubic centimeters per square foot ofsurface.

The invention also provides a metal article having a surface coated by the method or by means of the apparatus set forth above.

The invention is illustrated, merely by way of example, in the accompanying drawing which shows an apparatus which may be employed to coat a moving metal strip in accordance with the method of the present invention.

In the drawing there is shown an apparatus comprising an uncoiler 1 which carries a roll 2 of mild steel strip 3. The strip 3, which is uncoiled from the roll 2, passes over two spaced apart guide tables 4, 5. Between the tables 4, 5 there is disposed a shear 6 which may be operated, by means not shown, to shear the strip 3 as required.

After leaving the table 5, the strip 3 may pass along a cleaning line (not shown) which may comprise a degreasing bath, where the strip may be subjected to a degreasing liquid while being scrubbed, a cold water rinse, a pickling bath, and a further cold water rinse.

After passing through the cleaning line, the strip 3 passes through a bath 7, squeegee rolls 8, and sprays 9 which collectively constitute a wetting station. The bath 7 and the sprays apply to the opposite surfaces of the strip 3 a solution containin g sodium silicate (or, alternatively, potassium silicate) e.g. in a concentration of 1.0 to 5.0 (and preferably of 3) grams of sodium silicate per liter of water. The sodium silicate may be a commercial sodium silicate containing 18 percent w/w sodium oxide, and 36 percent w/w ofsilicon.

The bath 7 and squeegee rolls 8 are provided in order to ensure that the strip has already been thoroughly wetted with the sodium or potassium silicate solution before being sprayed therewith, whereby to obtain a more uniform film of the said solution on the strip.

The sprays 9 may subject the opposite surfaces of the strip 3 to the said solution in an amount from 0.75 to 2.5 and preferably about 1.5 cubic centimeters of the said solution for each square foot of the adjacent surface. This amount of solution is such as to trap the powder which is subsequently applied to the strip while being insufficient to cause pools of the solution to build up on the strip.

After leaving the wetting station 7-9, the strip 3 then passes through a first powder application station constituted by two powder deposition units 10'mounted above the strip 3. The powder deposition units 10 apply aluminum (or other metallic) powder to the upper surface of the strip 3. Each of the powder deposition units 10 comprises a porous meter roll 11 having a peripheral pockets (not shown) which are supplied with powder from a hopper 12. The powder in the said pockets is blown by a stream of air or other gas towards the upper surface of the strip 3. The powder passing towards the .upper surface of the strip 3 passes through electrostatically charged screens 13, the voltage applied to the screens being, for example, approximately 20,000 volts for each inch of distance between the bottom of the screens 13 and the strip 3.

The aluminum powder thus falls in an electrostatically charged condition onto the adjacent surface of the strip 3 where it would normally lose its charge and be re-attracted back upwardly were it not trapped by the sodium or potassium silicate solution on the strip. The potassium or sodium silicate, however, prevents oxidation or other corrosion of the aluminum powder by the water in the said solution.

The strip 3, which has been so coated, then passes above two powder deposition units 14 which constitute a second powder application station at which the aluminum (or other metallic) powder is applied to the lower surface of the strip. Each of the powder deposition units 14 comprises a porous meter roll 15 having peripheral pockets (not shown) which are supplied with powder from a hopper 16, the powder in the said pockets being blown towards the underside of the strip 3 by air or other gas reaching the inner surface of the meter roll 15 through a fluid conduit 17. The meter rolls 15 and strip 3 are earthed, the powder from the meter rolls 15 which is directed towards the strip 3 passing successively through electrically conducting screens 20, 21 of which the screen 21, which is that nearer to the strip 3, is maintained (by means not shown) at a negative or positive potential e.g. a potential of 30 kv. The screen 20, which is that nearer to the meter roll 15, is electrically isolated, but may have an electrostatic charge, e.g. ofl0 kv, induced in it.

After passing successively through the wetting station 7-9, the first powder application station 11, and the second powder application station 14, the strip passes to a first drying station 22, at which each ofthe surfaces ofthe strip is dried by passing the strip through a high frequency heater. As a result the sodium silicate remaining on the strip acts as a binder which prevents the aluminum powder from falling off the strip.

The strip 3 then passes between sprays 23, 24 which constitute a liquid application station at which the upper and lower surfaces of the strip are wetted with a liquid whose purpose is indicated below.

Thereafter, the strip 3 passes through a second drying station 25 and then a rolling mill 26 which constitutes a compaction station, the rolling mill 26 having rollers 27, 28 between which the strip passes and which compact the aluminum coating to both the surfaces of the strip. The strip 3 is finally coiled onto a roll 30 of a coiler 31 from which it is removed to be heat treated at an elevated temperature e. g. by being heated at 500 C. for 1 hour, or by being heated by 350 C. for 15 hours.

The aluminum powder applied to the strip is typically that which passes a 300 mesh sieve. The powder is dispensed by the powder deposition units 10 such that the powder contains a high percentage offines e.g. the powder contains 30 to 40 percent of particles less than 20 microns and 10 to 15 percent of particles less than l0 microns.

While this powder gives a good quality coating to the strip, it has very poor flow properties. Consequently, if the strip is passed through the rollers 27, 28 at more than a moderate speed the powder is dislodged and banks of dislodged powder build up immediately ahead of the rollers.

It is to combat this problem that the sprays 23, 24 are provided. By means of the sprays 23, 24, a liquid is applied on top of the layer of powder on the strip, the liquid being such as to reduce the extent to which the rollers dislodge the powder.

The liquid applied to the strip by the sprays 23, 24 may be an aqueous colloidal solution of a substance which gels when suitably hydrated. Examples of such substances are nickel hydroxide, aluminum hydroxide, ora water soluble cellusic material such as sodium carboxy methyl cellulose.

As a result of the wetting by the liquid from the sprays 23, 24, the strip may pass at, for. example, 60 ft. per minute or even 100 ft. per minute, whereas if this wetting is omitted, it may be necessary to limit the speed of the strip to, say, 30-35 ft. per minute. r

The sprays 23, 24 may be such as to apply from 2.0 to 4.0 (and preferably about 3.0) cubic centimeters per square foot of solution to each side of the strip. When sodium carboxy methyl cellulose is employed, the concentration thereof in the said solution should be within the range of about 0.05 to 0.6 percent by weight of the solution. Preferably, the concentration is not less than 0.2 percent nor more than 0.4 percent by weight.

When aluminum hydroxide or nickel hydroxide is employed, the concentration of the solution preferably is within the range 0.05 to 1 percent by weight. Another substance that has been successfully used is starch in an aqueous colloidal solution of a concentration not less than 0.2 percent nor more than l percent by weight. The-concentration is preferably 0.5 percent by weight.

Bentonite is also satisfactory when used in an aqueous colloidal solution of concentration not less than 0.5 percent nor more than 2 percent by weight. The concentration is preferably l percent by weight.

The following,experimental results illustrate the effect of applying a suitable liquid via the sprays 23, 24.

l/\ lilll'i l Results ()lltitlllttl without the application of a suitable liquid via sprays .33, 21]

Adhesion of Maximum layer after strip speed compaction Pro-wetting by sprays 9 on steel prior to through and heat powder application rolls, lt./min. treatment Water, 1.5 (Jo/sq. ft Good. Water, 4 era/sq. it. 17 Do. Water, 6 co./sq. ft 3 gins/litre of aqueous solution of sodium Good silicate 2 cc./sq. ft. 6 gins/litre of aqueous solution of sodium Fair.

silicate, 2 ce./sq. it. 0.2% sodium carboxy methyl colluslose so-' 40 Do.

lntion-Zco/sq. it 0.3% sodium carboxy methyl cellulose 60 Poor.

tion, 2 cc./sq.'lt.

0.5% sodium carboxy methyl cellulose solution. 2 (tn/sq. l't.

. 'loo wet. powder runs into pools. Speed limit of plant.

Steel strip=0.0t0 thick coated on both sides with the aluminum powder to produce a compacted coating 40 mireons thick.

instead of sodium silicate. The maximum concentration of such as indicated. previously was sprayed on top of the dried sodium carboxy methyl cellulose acceptable is about 0.2 per- 7 cent by weight. However, itcan be seen that the use of sodium carboxy methyl cellulose has the same disadvantages as indicated for sodium silicate. We have found however that increased concentrations of sodium silicate or particularly sodium carboxy methyl cellulose may be advantageous in that the layer of powder is less likely to be dislodged before compaction due to vibration ofthe steel strip. Y I

We think that the more concentrated solutions applied via the sprays 9 cause poor adhesion of the coating to the subpowder coating, by means of the sprays 23-, 24 mentioned above. Table 2 indicates the results obtained.

TABLE 2 [Results obtained with the application ola suitable liquid via sprays 23, 24]

Adhesion of Pro-wetting by Maximum layer after sprays 0 on steel strip SI)l((i ('(illllitll'tllill prior to powder Post-wet liquid applied through rolls and heal. application by sprays 23, 24 l't./miu. tri-utunut Water 0.2% sodium carboxy 155 Good.

methyl cellulose, 3 cor/Sq. it. Do 0.4% sodium carboxy 60 Do.

methyl cellulose, 3 cc./sq. it. Sodium Silicate." 0.2% sodium carboxy 40 Do.

methyl cellulose, 3 oe./sq. It. Do 0.4% sodium carboxy Do.

methyl cellulose, 3 ce./sq. it. Do 0.5% colloidal solution 50 Do.

ofstarch 3 ecu/sq. it. Do 1% colloidal solution of 5') 1m.

Bentonite 3 cc./sq. it. Do 0.5% colloidal solution (in lie.

nickel hydroxide, 3 (.(L/Sf]. it.

Steel strip=0.040 thick coated on both sides with tho nlunu'uiuu powder to produce a compacted coating 40 microns thick.

We are of the opinion that the powder layer acts as a filter for the solutions applied via the sprays 23, 24. Two mechanisms appear possible. Either the powder layer absorbs water from the solutions so that the remaining solution either gels or becomes too viscous to flow to the layer-to-substrate interface. Alternatively, the powder layer filters out the solids in the solution and allows only water to approach the interface.

We think that the filtered-out or gelled material at the surface of the powder layer glazes the surface thereof to bind the powder particles together.

It is desirable both to dry the strip, at the first drying station 22, prior to having the said liquid applied to it, and also to dry the strip at the second drying station 25 prior to the strip passing to the compaction station 26.

Other liquidswhich should prove to be successful are solutions containing gum arabic or gum treganth or naturally oc- 7 content is required to produce high viscosity. This means that the amount of contaminant in the upper layers of the powder coating is relatively small and will not prevent the compacted powder from sintering. I i

As will readily be seen from the drawing, the wetting station I -9, the first powder application station 11, the second powder application station 14, the first drying station 22, the liquid application station 23, 24, the second drying station 25, and the compaction station 26 are absolutely in line with each other so that the strip 3 when passing through these stations does not have its direction of travel altered. Furthermore, it will also be seen that the uncoiler l and the coiler 31 are also substantially in line with the various stations 7-9, ll, 14, 22, 23, 24, 25 and 26. This arrangement reduces the number of guide rolls over which it is necessary for the strip 3 to pass, and therefore reduces the extent to which dirt can readily be picked up by the strip 3. It is very desirable to limit transference of dirt to the strip, since such dirt can have an adverse effect upon the adhesion developed between the coating and the strip during the final heat treatment.

The alignment of all the various stations, moreover, reduces to a minimum the amount of strip which is passing through the apparatus at any one time, this being desirable if high losses are to be prevented during temporary shut-downs of the apparatus.

It will be appreciated that the invention can be employed to coat moving metal surfaces other than the surfaces of a strip. For example, wire or rod can be similarly coated.

We claim:

1. A method of coating a moving metal surface comprising the steps of forming a dry layer of metallic powder on the surface, thereafter applying to the said layer, an aqueous liquid of a substance which gels when suitably hydrated and allowing the water thereof to penetrate the metallic powder layer, and then passing the metallic powder layer at a speed of at least 35 feet per minute against a compression roller to compact the powder to the said metal surface, the said liquid acting to reduce the extent to which the roller dislodges the powder.

2. A method as claimed in claim 1 wherein the said surface I is the surface of a moving metal strip, the powder being compacted by passing the strip through the nip of a pair of cooperating said compression rollers.

3. A method as claimed in claim 1 comprising drying the layer after the liquid is applied, before passing it against the said roller.

4. A method as claimed in claim 1 wherein the said liquid is an aqueous colloidal solution.

5. A method of coating as claimed in claim 4 in which the liquid is an aqueous colloidal cellulosic solution.

6. A method of coating as claimed in claim 5 wherein the liquid is an aqueous solution containing sodium carboxy methyl cellulose.

7. A method as claimed in claim 6 wherein the concentration of the sodium carboxy methyl cellulose in the solution is not less than 0.05 percent nor more than 0.6 percent by weight.

8. A method as claimed in claim 7 wherein the said concentration is not less than 0.2 percent nor more than 0.4 percent.

9. A method as claimed in claim 4 wherein the solution contains starch, at a concentration between 0.2 and 1 percent by weight.

10. A method as claimed in claim 4 wherein the solution contains bentonite at a concentration of not less than 0.5 percent nor more than 2 percent by weight.

11. A method as claimed in claim 4 wherein the solution contains aluminum hydroxide at a concentration of not less than 0.05 percent nor more than 1 percent by weight.

12. A method as claimed in claim 4 wherein the solution contains nickel hydroxide at a concentration of not less than 0.05 percent nor more than 1 percent by weight.

13. A method as claimed in claim 1 wherein the liquid is applied to the surface at a rate between 2 and 4 cubic centimeters per square foot of surface. 

2. A method as claimed in claim 1 wherein the said surface is the surface of a moving metal strip, the powder being compacted by passing the strip through the nip of a pair of co-operating said compression rollers.
 3. A method as claimed in claim 1 comprising drying the layer after the liquid is applied, before passing it against the said roller.
 4. A method as claimed in claim 1 wherein the said liquid is an aqueous colloidal solution.
 5. A method of coating as claimed in claim 4 in which the liquid is an aqueous colloidal cellulosic solution.
 6. A method of coating as claimed in claim 5 wherein the liquid is an aqueous solution containing sodium carboxy methyl cellulose.
 7. A method as claimed in claim 6 wherein the concentration of the sodium carboxy methyl cellulose in the solution is not less than 0.05 percent nor more than 0.6 percent by weight.
 8. A method as claimed in claim 7 wherein the said concentration is not less than 0.2 percent nor more than 0.4 percent.
 9. A method as claimed in claim 4 wherein the solution contains starch, at a concentration between 0.2 and 1 percent by weight.
 10. A method as claimed in claim 4 wherein the solution contains bentonite at a concentration of not less than 0.5 percent nor more than 2 percent by weight.
 11. A method as claimed in claim 4 wherein the solution contains aluminum hydroxide at a concentration of not less than 0.05 percent nor more than 1 percent by weight.
 12. A method as claimed in claim 4 wherein the solution contains nickel hydroxide at a concentration of not less than 0.05 percent nor more than 1 percent by weight.
 13. A method as claimed in claim 1 wherein the liquid is applied to the surface at a rate between 2 and 4 cubic centimeters per square foot of surface. 